Magnesium aluminate spinel, MgAl2O4, (hereinafter “spinel”) is a very attractive ceramic material for use in various applications requiring a rugged, tough, scratch resistant, transparent material. Examples include both military applications such as protective windows for aircraft and armored vehicles, armor, missile domes, protective goggles, ruggedized computers and displays, and laser weapons, as well as commercial applications, such as fixed and portable point of sale (POS) terminals, watch crystals, vapor lamp tubes, firefighter and police face shields, laptop computers, cell phones, automotive glassing and headlamps, industrial blast shields, and transparent orthodontic fixtures. Spinel articles have a wide transparency range from visible to 5.5 μm wavelength, and mechanical properties several times greater than that of glass while being remarkably lighter than ballistic glass by a factor of 2 for the same degree of armor ballistic protection.
The consolidation of commercially available high purity spinel powder into dense, highly transparent monolithic shape parts, is carried out through a sintering process at high temperature. The sintering process may be pressure assisted as in hot pressing, or it may be pressureless. In order for spinel powder to be effectively sintered, it must be intimately mixed prior to sintering with very small amounts of particles of an inorganic sintering aid, such as lithium fluoride (LiF), which helps in reducing and homogenizing the grain size and eliminating porosity in the sintered product. As illustrated by such prior art patents as the Sellers et al. U.S. Pat. No. 3,768,990, issued Oct. 30, 1973, the Roy et al. U.S. Pat. No. 4,930,731, issued Jun. 5, 1990, the Roy et al. U.S. Pat. No. 4,983,555, issued Jan. 8, 1991, and the Roy et al. U.S. Pat. No. 5,244,849, issued Sep. 14, 1993, this has traditionally been accomplished by some form of mechanical mixing of the sintering aid particles with the spinel powder, such as a mortar and pestle, ball milling, attritor milling or high shear wet milling.
There have been numerous attempts to develop an economically viable plant scale manufacturing process for the commercial production of dense, highly transparent, defect-free spinel articles. The inability to achieve high yields of uniformly transparent shapes has been a nagging problem thwarting these attempts. All too often, the final products exhibit opaque or hazy regions, leading to high rejection rates. These product defects have been found to be due primarily to inhomogeneous mixing of the sintering aid particles with the spinel powder prior to sintering, resulting in non-uniform distribution of sintering aid during the sintering process. Adding to this inhomogeneity problem is a contamination problem caused by the mechanical mixing techniques themselves, since even low ppm levels of impurities from the abrasion of the grinding media can cause opaque or hazy regions in the final product.
The foregoing problems associated with the mechanical mixing of the sintering aid particles with the spinel powder prior to sintering have been addressed by Villalobos et al. in U.S. Patent Application Publication No. 2004/0266605, published Dec. 30, 2004, U.S. Pat. No. 7,211,325, issued May 1, 2007, and U.S. Pat. No. 7,528,086, issued May 5, 2009. The solution proposed by Villalobos et al. is to form a coating of the sintering aid on the spinel particles by the spray drying of a dispersion of spinel particles in a solution of the sintering aid. While the Villalobos et al. spray drying approach does achieve a more uniform distribution of sintering aid within the spinel powder, it does so at the expense of a significant sacrifice in spinel powder yield, which has been found to be no better than about 68%. This means that up to 32% of spinel powder processed through the spray dryer will wind up being lost. This low production yield, coupled with possible contamination added during spray drying derived from solution pumping, dryer heater, hot air contamination and contact with the metallic components of the spray dryer, all tend to seriously detract from the commercial appeal of this approach.
It should be noted that throughout the above-referenced U.S. Patent Application Publication No. 2004/0266605, Villalobos et al. specifically caution against allowing precipitation of the dissolved sintering aid among the spinel particles in the dispersion to be spray dried. See, for example, paragraphs 0017, 0027, 0030 and 0044.